The present invention relates to an apparatus for controlling the parallel operation of an A-C output converter and a commercial power source, in order to supply the electric power or to store the energy while operating the A-C output converter in parallel with the commercial power source in a power failure-free power source apparatus, in a solar generating system, in a fuel cell generating system or in a secondary cell energy storage system.
FIG. 5 shows a conventional apparatus of this kind, wherein reference numeral 1 denotes an inverter which is operated in parallel with a commercial power source 2 via a commercial bus 3 to supply electric power to a load 4. The inverter 1 is chiefly comprised of an inverter unit 100, an output transformer 101, a filtering reactor 102 and a filtering capacitor 103. The inverter 1 converts the electric power of a D-C power source 5 into an alternating current and is connected to the commercial bus 3 via an output switch 104.
The operation will be described hereinbelow. To operate the inverter 1 in parallel with the commercial power source 2, first, a detect signal I.sub.10 is obtained by a current transformer 106 from an output current I.sub.1 of the inverter 1. Then, two voltage vectors E.sub.A and E.sub.B that meet at right angles are obtained by a phase shifter 108, and are supplied to a Q detecting circuit 109 and a P detecting circuit 110, respectively, to obtain a component Q that corresponds to the reactive power and a component P that corresponds to the active power from the output current I.sub.1 and the detect signal I.sub.10. Relying upon the signals from a voltage setting circuit 111 and a voltage feedback circuit 112, the inverter 1 modulates the pulse width of the inverter unit 100 via a voltage control circuit 113 and a pulse width modulation (PWM) circuit 114, in order to control the internally generated voltage.
A subtracting circuit 117 finds a difference between the component Q that corresponds to the reactive current and a setpoint value Q.sub.R of the reactive power to be produced. The difference is amplified by a Q control circuit 116 and is supplied as an auxiliary signal to the voltage control circuit 113, in order to adjust by about several percent the internally generated voltage of the inverter unit 100 so that the component Q that corresponds to the reactive power is brought into agreement with the setpoint value Q.sub.R of the reactive power.
In the inverter 1, furthermore, a phase difference .DELTA..phi. detecting circuit 124 detects a phase difference .DELTA..phi. between the commercial bus 3 and the internally generated voltage of the inverter. A PLL amplifier 115 adjusts the frequency of a reference oscillator 105 so that it is brought into synchronism with the commercial power source 2, and that the phase difference is brought into zero. A subtracting circuit 119 finds a difference between the component P that corresponds to the active power and the active power P.sub.R that is to be produced. The difference is amplified by a P control circuit 118 and is supplied as an auxiliary signal to the PLL amplifier 115 in order to finely adjust the phase difference between the inverter 1 and the commercial power source 2, so that the active power P being produced is brought into agreement with the setpoint value.
As described above, the inverter 1 and the commercial power source 2 are operated in parallel to deal with the active power and the reactive power, and to stably carry out the operation.
With the conventional apparatus for controlling the parallel operation of the A-C output converter and the commercial power source constructed as described above, however, it is quite difficult to test and adjust the parallel operation. For example, to test and examine whether the inverter system to which the parallel operation system is adapted properly operates as expected or not, it is necessary to connect the inverter 1 to the commercial bus 3 to practically operate it. As is widely known, however, the inverters in general have an overcurrent withstand capacity of only about 150%. Therefore, it is quite difficult to examine the presence of any abnormal condition in the control circuit or to adjust the response characteristics of control while practically operating the system of FIG. 5.
In practice, therefore, operation of the whole system of FIG. 5 is tested after the individual elements in the control circuit of FIG. 5 are completely tested and adjusted, and after it has been confirmed that there is no error in the wirings among the elements. Even when sufficient attention is given to perform the parallel operation, however, an excess of transverse current flows due to unexpected faults, and the inverter often undergoes commutation failure and is damaged. This means that a very cumbersome operation is required to investigate the occurrence of trouble (particularly troubles such as a poor contact that develops when the reproduceability is not good) and to carry out regular maintenance.